We should be alarmed about rising sea levels

Science fiction or future reality? A scene from the film FloodPhoto: Lionsgate Films

By Fred Pearce

7:27PM GMT 11 Mar 2009

The oceans are stirring. Deep beneath the sea, primeval forces have been unleashed that may engulf us all – unless we take to our boats to escape the rising tides.

That, as it happens, is the plot of John Wyndham's science fiction classic of the 1950s, The Kraken Wakes. But it also seems disturbingly close to what some scientists are now predicting. Once, rising sea levels were seen as a threat to distant coral islands: toodle-oo for Tuvalu, goodbye for the Gilbert Islands and curtains for the Carterets. But this week scientists raised their forecast of the rise in 21st-century sea-level from about a foot to about a yard.

On the face of it, this didn't seem too much of a change: what's a couple of feet between friends? But it means we're talking about the exit of Essex, the fall of the Fens and Trent Valley turning turtle. And how many takers are there in Europe for the tens of millions of environmental refugees who would flee submerged river deltas such as the Nile and Ganges, or low-lying coastal areas in western Africa and eastern China?

This might seem like a scare story, and there is certainly controversy about what is actually happening. But some things are self-evident. If global warming carries on as predicted, it is bound to raise sea levels, simply because warmer water takes up more space. The old forecast allowed for this, but a second element has been added, in that the ice in polar regions is melting much faster than expected. As with an ice cube in a glass of water, floating ice doesn't change sea levels as it melts – but dumping in more ice, in the shape of the huge ancient sheets that cover Greenland and Antarctica, would certainly have an impact. The question is how much, and how soon?

Sea levels had already been rising for almost 400 years, as the world recovered from the Little Ice Age. That much was perfectly natural. But the process has been accelerating: a two-centimetre rise during the 18th century became six centimetres in the 19th century and 19cm in the 20th. In the 1990s, the rate rose by 50 per cent, reaching the equivalent of 30cm in a century, and is now nearer 40cm. If things continue, a one-metre rise by 2100 looks more than likely: indeed, the Thames Barrier is already being raised more and more often to protect London, and some studies suggest a new and bigger barrage could be needed as soon as 2030.

Another cause for concern is that ice piled up in Greenland and Antarctica, in layers up to one and a half miles thick, is shifting uneasily. "Glacial" used to mean "very slow": but it now looks as if, rather than melting gradually from the top down, over thousands of years, these sheets will break up long before they have a chance to melt, cascading into the oceans in the form of a million icebergs.

Over a couple of centuries, this could raise sea levels by six metres: a seemingly unstoppable process that may already be under way. Last summer, scientists reported from a region they call Greenland's Lake District, on a pool of meltwater two and a half miles across, which disappeared down a crack in the ice in just 90 minutes. The flow was greater than Niagara Falls, forming a river beneath the ice that lifted the entire ice sheet up by half a metre and moved it towards the ocean.

Nobody knows how new or extensive such processes are. But the scientists think they explain why Greenland is currently chucking far more ice into the ocean than it gains in snowfall, at ever faster rates. One of the biggest exit points is the Jakobshavn glacier, which once gave the world the iceberg that sank the Titanic.

Meanwhile, at the other end of the planet, half of Antarctica also seems to be in trouble. The Nasa glaciologist Eric Rignot reported last year that the West Antarctic ice sheet was losing ice twice as fast as in the 1990s. Again it has an Achilles' heel – a place known as Pine Island Bay, where two huge glaciers converge.

The West Antarctic sheet is not even on solid land. It is perched, like a huge wrecked ship, on an archipelago of submerged islands. Thirty years ago, an eccentric British glaciologist called John Mercer, best know at the lab for doing his field work in the nude, wrote a paper on the sheet for Nature magazine. He forecast that it might one day break free, raising sea levels by five metres within a few decades. They used to laugh: not now.

There is, of course, the possibility that the glaciologists, unaccustomed to the limelight, are getting carried away by their own discoveries. Some scientists caution that the physical break-up of ice sheets may be self-limiting: the big rivers beneath the ice may quickly re-freeze and the threatened flotillas of icebergs may stay at home.

Sceptics will also wonder whether we can believe boffins who can't get their predictions right. Two years ago, they said sea-level rise this century would range from 18cm to 59cm. This week, a glaciologist called Konrad Steffen, of the University of Colorado, told a climate science summit in Copenhagen that the new range stretched from 90cm to 120cm. Why believe them this time?

It's a fair point, but how lucky do we feel? After all, even faster changes have happened before when the world warmed: wash-marks around the planet's shorelines show that towards the end of the last ice age, about 14,500 years ago, sea levels rose by about 20 metres in 400 years, or five metres for each of four centuries. And worryingly, the last time the planet was as warm as it is today, about 120,000 years ago, the oceans were six metres higher than today. Indeed, some say the Greenland ice sheet is a relic of the last ice age, already past its melt-by date.

And the danger is clear: even a one-metre rise would flood the homes of around 100 million people in Asia, mostly in eastern China, Bangladesh and Vietnam. Around 14 million Europeans would be threatened around the shores of the North Sea, and eight million each on the west coast of Africa and the northern shores of South America. Megacities such as Shanghai, Karachi, Lagos and Bangkok would have to spend tens of billions of dollars on protecting themselves, or be submerged. Towns along the US eastern seaboard would mostly go too, including great chunks of Florida. New Orleans and Venice would disappear. The Netherlands reckons it can protect itself – at a cost of $5 trillion. But Britain, which has one of the longest coastlines in the world, may not even try. Low-lying areas vulnerable to rising tides – and the increased storminess that climatologists also predict – include East Anglia, the Wash, the Somerset Levels, the Trent Valley and Romney Marsh.

As bad luck would have it, geological forces mean some of these areas are already sinking, which will add around 25cm to the effective sea level round East Anglia this century. (If you are thinking of selling that cottage in the Norfolk Broads, remember that north-west Britain is rising by about the same amount, so should stay above the waves rather longer.)

The Environment Agency wants to give up some areas without a fight: towns and industry will get priority for protection, it says, but much farmland will be abandoned. Ely was once an island in the Fens, and could be again. In other words, the Kraken may indeed be waking.

Fred Pearce is environment consultant of 'New Scientist', and the author of 'The Last Generation: How Nature Will Take Her Revenge for Climate Change' (Transworld), which is available from Telegraph Books for £8.99 + 99p P&P. To order, call 0844 871 1515 or go to books.telegraph.co.uk